sorted the porsche gluing to the track problem, turned the magnet over which has made it much more fun for both of us as i run the outer lane when we're side by side he wipes me out with the 911 rear end. we are having much more fun than driving "glued" cars
I know that when I took my Stillen GT-R out and ran it on Scaley's new tires, it held pretty good and then when I went with Super Tires, they had a lower profile and the magnetic effect was increased. I compared it to my Ferrari 430 that was running in the "black window" class here and the Magnet Marshall said they were within 10 grams of each other (GT-R being lighter) and the GT-R ran better times because the motor was a real punchy sweet one (of course the Ferrari has many many laps on it too) The GT-R gives the Lambos (car of choice by many here) a real run for the money and I opened it up to all to see that there was no monkey business going on. Now, if I could only drive well.
It's possible that it's not actually understeer. Could be that with the front screw loose, there could be some slight tire rub or other issue caused by the loose front. Check for any kind of rubbing or interference in the front end of the car.
the more you loosen the body, the more tendency for the car to roll out of the slot vs. fishtail. since scaley cars have a fairly shallow guide flag, it doesn't take much body roll before the car "understeers" out of the slot and goes straight instead of turning.
mess around with the screws until you find the sweet spot where the car is stable and will slide predictably at the limit rather than roll/understeer. i usually start with the front screws about 1/4 turn off tight and rear screws about 1 turn off, and make adjustments from there.
With your screws, use them like suspension, ie tight is stiff, loose is soft- make sense? I read 'how to make your car handle' for reference, and suspension principles apply even in our 75 gram rockets.
When you first had your issue, should you have been running sans magnet then loosening the rear screws would help it to turn with reduced oversteer.